1. Field of the Invention
The present invention relates generally to a pointing device adapted for various coordinate input devices such as a digitizer or a tablet which have been well known as one of the input units of a computer system. Especially, the present invention relates to a pressure sensitive element and a stylus pen using the pressure sensitive element.
2. Description of the Prior Art
Conventional stylus pens used as pointing devices for a digitizer have been constituted for detecting pen-down information in addition to coordinate information on the pointing device. Such pen-down information represents one specific condition: that of the pen point being in contact with a panel of the digitizer. The pen-down information is detected by pressure sensitive means for detecting the vertical pressure applied to the pen point.
As a typical point detecting system for a digitizer, an electromagnetic signal utilizing system has been commonly used. This system comprises a point detecting panel including a plurality of sensor coils arranged in parallel in the detecting direction, and a pointing device such as a pen or a cursor. Since the pointed coordinate information is represented by X and Y coordinates, the detecting area of the sensor panel is composed of two sections intersected perpendicularly in X and Y directions. The pointing device is provided with a coil or resonant circuit to perform electromagnetic interaction between the pointing device and the sensor coils. A typical detecting operation of such constitution will be explained. The sensor coils in the sensor panel transmit electromagnetic waves and then receive response electromagnetic wave re-emitted by the interaction between the transmitted electromagnetic waves and the resonant circuit in the pointing device. The coordinate values of the pointing device are produced by arithmetic operation according to a main sensor signal generating the strongest electromagnetic energy and supplementary sensor signals adjacent to the main sensor.
An electromagnetic signal utilizing system constituted as such provides some advantageous features one of which allows the pointing device to be used without a connection lead; i.e., cordless. The applicant of the present invention has already provided various digitizers according to this electromagnetic signal utilizing system as shown in Japanese Patent Publication No. 2-53805/1990 and Japanese Patent Application Open-Publication No. 3-147012/1991.
FIG. 9 shows a stylus pen type pointing device 300 disclosed in these publications. In detail, the stylus pen 300 includes an external casing 302, as a pen holder, formed in a cylindrical shape made of non-metal material such as plastics, and internal members contained in the casing 302. The internal members include, from the bottom to the top in the drawing, a core member 304; a core support member 306 composed of a ferrite core with a through hole in which the core member 304 is slidably supported and a coil wound around the circumferential surface of the ferrite core; a coil spring 308; a switching member 310; and capacitors 312 and 314. The top end of the casing 302 is provided with a cap 316. The coil 306 and the capacitors 314 define a parallel resonant circuit. The physical properties of these elements of this resonant circuit are previously selected to resonate with the electromagnetic wave from the sensor coils arranged in the sensor panel.
The capacitor 312 is connected to both ends of the coil 306 through the switching member 310 so that the capacitor 312 will act to delay the phase of current flowing through the resonant circuit. When the forward end of the core member 304 is forcibly put in contact with the sensor panel (this operation is referred to as "pen down"), the core member 304 is moved backwards into the interior of the pen holder 302. The rear end of the core member 304 push the switching member 310 backwards through the coil spring 308. Thus the switching member 310 is turned on to change the phase of the response electromagnetic wave generated by the resonant circuit. This change in the phase represent a switching information.
There has been known another stylus pen having a pen pressure sensing function to sense the pressure an operator puts on the pen (hereinafter, referred to "pen pressure") rather than the above described simple on-off switching mechanism. Such pen pressure sensing function is provided by replacing a part of the capacitor or coil of the above described resonant circuit with a variable capacitor or variable coil, or adding a variable resistor to the resonant circuit. These variable elements are easily varied in response to the pen pressure. Such pen pressure sensing type stylus pen is disclosed in Japanese Patent Application Open-Publications No. 63-257823/1988, No. 64-53223/1989, No. 4-96212/1992, No. 5-275283/1993. As the variable resistor, a pressure sensitive conductive rubber has been suggested. However, the pressure sensitive conductive rubber with a desirable response and reproduction ability has not been produced which would give the stylus pen with the required properties.
The core member 304 for the above described conventional stylus pen is not limited to a plastic rod, but any conventional pen point units such as ball point pen and mechanical pencil may also be applied to the core member. These conventional pen point units can also be used to write or mark any required characters or illustrations on the paper sheet laid on the sensor panel in addition to the pen down operation.
For example, FIG. 10 shows a typical structure of the inner unit 200 of ordinarily used mechanical pencils. This type of mechanical pencil is operated by a pushing motion to thrust the pencil point, and further provided at the rear end with a lead feeding means through which supplementary leads are fed. The specific type of mechanical pencil and ball point pen having this push and thrust function are referred to as "push and thrust type writing items" in the present specification. FIG. 10(a) shows one typical example of the knock operative inner unit 200 for a conventional mechanical pencil, commercially available. The inner unit 200 includes a spring actuator which is not shown because it is installed in a lead thrust mechanism 84, and a lead reservoir 86 for reserving spare leads. When the rear end 87 of the lead reservoir 86 is pushed towards the lead thrust mechanism 84, a thrust rod 85 is slidingly moved about 3 mm into the lead thrust mechanism. This thrust motion allows the lead point 112 to move forwards at a predetermined length.
FIG. 10(b) shows an assembled state of the inner unit 200 shown in FIG. 10(a). The inner unit 200 is installed in a pen holder 80 and provided at the rear end with a push button 83 protruded outwardly through a rear opening of the pen holder 80. When a user pushes the push button while holding the pen holder 80, a stepped portion 82 of the thrust mechanism 84 is brought into contact with the tapered inner wall of the pen holder 80 and then the thrust rod 85 is slidingly moved into the lead thrust mechanism 84. Accordingly, the point 112 is protruded outwardly through a front opening of the pen holder 80. The user holds the pen holder 80 and applies the pressure to the point 112 onto a paper sheet to write characters or marks.
In such conventional push and thrust type writing items, the push and thrust or end cap must be removed to supply supplementary leads or replace the inner unit with a new one through the rear opening of the pen holder. As described above, it is advantageous that a conventional writing item is used as a stylus pen. However, it is impossible to combine the core unit of the push and thrust type writing item within a conventional stylus pen because such a conventional stylus pen includes a switch and electronic elements for a resonant circuit which occupy most of the inner space of the pen holder 302 as shown in FIG. 9. This means it is impossible to arrange the lead reservoir or means for replacing the core unit in the rear portion of the stylus pen.
Furthermore, even if the core unit of the push and thrust type writing item is installed in such a conventional stylus pen, it would be impossible to arrange a means for sensing the pressure applied to the point along the longitudinal axis of the stylus pen.